Passive way to improve heat distribution in packed bed storage

ABSTRACT

A system for storing heat is provided having a first housing section with a first storage volume, wherein the first housing has a first opening for connecting the first storage volume to an environment. A first type of first heat storing elements is arranged within the first housing section. The system has a second housing section with a second storage volume, wherein the second housing section has a second opening for connecting the second storage volume to the environment, wherein the first and second storage volumes are interconnected for forming a common storage volume. A second type of second heat storing elements is arranged within the second housing section. The first heat storing elements of the first type have a respective first element size, wherein the second storing elements of the second type have a respective second element size. The first element size differs from the second element size.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of European Application No.EP14164954 filed Apr. 16, 2014, incorporated by reference herein in itsentirety.

FIELD OF INVENTION

The present invention relates to a system for storing heat and to amethod for manufacturing a system for storing heat.

ART BACKGROUND

Nowadays, to improve the efficiency of power plants on the one hand andto smooth network fluctuations caused by the time dependent electricalconsumptions on the other hand, integration of thermal energy storagesin the power plant system plays a role of increased importance. Severaltypes of thermal energy storages developed recently are available today.

Thermal energy storages are subdivided into the following classes. Solidbed storages which use heat capacity of filled material, liquid thermalstorages which use a phase change enthalpy and storages which usechemical reaction enthalpy could provide required storing ability ofthermal energy.

The solid bed storages are filled with heat storing elements, such asstones, bricks or ceramic parts which are a cost effective solution.

The storage dynamics during the charging and discharging phase and thestorage efficiency can be improved using preventive measures reducingnegative influence of natural and forced convection effects occurring inthe storage medium. These effects are taking place during the wholeprocess cycle, i.e. in the charging phase, in the quasi-steady-state(only natural convection) and in the discharging phases. A non-linearimpact of natural and forced convection during the charging anddischarging phases is resulting in a declination of isothermals in thestorage medium. The hot working fluid has a lower density which migratesto the top layers of the heat storage and thereby leads to a change inthe temperature field in the storage medium on the one hand anddistribution or declination of isothermals on the other hand.

Construction features (e.g. large cross section area of storage medium)and engineering features (e.g. design of the incoming flow) requiresegmentation of the incoming flow of working fluid into multiple streamsof the working fluid. Hence, segmentation of the mass flow rate over thefull cross section of the thermal storage area is necessary. Thedistribution of the total mass flow rate of the incoming working fluidinto the storage depends on the selected geometry of the manifold whichis to be used.

The declination of the angle of the isothermals in the storage medium tothe flow direction of the working fluid is strongly dependent on thebuilding/storage height and leads to an increase of temperaturediscrepancies in the inlet and outlet sections of the storage during thedynamic phases and thus to a decrease of the average outlet temperatureleaving the system through the outlet section. This effect increases inthe case that the storage designed for segmentation of the working flowhas multiple inlets or outlets distributed vertically over the crosssection of the storage.

SUMMARY OF THE INVENTION

It may be an objective of the invention to provide a heat storage withproper internal heat distribution.

This objective is solved by a system for storing heat and by a methodfor manufacturing a system for storing heat according to the independentclaims.

According to a first aspect of the present invention, a system forstoring heat (i.e. thermal energy) is presented. The system comprises ahousing with a storage volume, wherein the housing comprises an openingfor connecting the storage volume to an environment. The system furthercomprises a first type of first heat storing elements which is arrangedwithin a first location within the storage volume and a second type ofsecond heat storing elements which is arranged within a second locationwithin the storage volume. The first heat storing elements of the firsttype comprise a respective first element size, wherein the secondstoring elements of the second type comprise a respective second elementsize. The first storing element size differs from the second elementsize.

According to a further exemplary embodiment, the housing section furthercomprises a further opening for connecting the storage volume to theenvironment, wherein the further opening is spaced apart from theopening. The first type of first heat storing elements is arrangedwithin the opening forming the first location, wherein the second typeof second heat storing elements is arranged within the further openingforming the second location.

According to a further exemplary embodiment, the housing section furthercomprises a center section arranged between the opening and the furtheropening, wherein an additional further type of additional further heatstoring elements is located within the centre section. The additionalfurther storing elements of the additional further type comprise arespective additional further element size, wherein the additionalfurther heat storing element size differs from the first element sizeand/or the second element size.

According to a first aspect of the present invention, a system forstoring heat (i.e. thermal energy) is presented. The system comprises afirst housing section and the storage volume comprises a first storagevolume, wherein the first housing comprises a first opening forming theopening for connecting the first storage volume to an environment. Thesystem further comprises a first type of first heat storing elementswhich is arranged within the first housing section forming the firstlocation. The housing further comprises a second housing section and thestorage volume comprises a second storage volume, wherein the secondhousing section comprises a second opening for connecting the secondstorage volume to the environment. The first storage volume and thesecond storage volume are interconnected for forming a common storagevolume. The system further comprises a second type of second heatstoring elements which is arranged within the second housing sectionforming the second location. The first heat storing elements of thefirst type comprises a respective first element size, wherein the secondstoring elements of the second type comprise a respective second elementsize. The first element size differs from the second element size.

According to a further aspect of the present invention a method formanufacturing a system for storing heat is presented. According to themethod, a first housing section with a first storage volume ispresented, wherein the first housing comprises a first opening forconnecting the first storage volume to an environment. A first type offirst heat storing elements is provided and the type of first heatstoring elements is arranged within the first housing section. A secondhousing section with a second storage volume is provided, wherein thesecond housing section comprises a second opening for connecting thesecond storage volume to the environment. The first storage volume andthe second storage volume are interconnected for forming a commonstorage volume. A second type of second heat storing elements isprovided. The second type of second heat storing elements is arrangedwithin the second housing section. The first heat storing elements ofthe first type comprise a respective first element size and the secondheat storing elements of the second type comprise a respective secondelement size. The first element size differs from the second elementsize.

The system according to the present invention comprises the housingwhich is formed of the first housing section and the second housingsection. The first housing section and the second housing section maycomprise a rectangular or elliptical cross section. The housing sectionsmay be made of metal material, for example. The housing sections aresurrounded by an environment, wherein the housing sections comprisematerial and/or an additional insulation layer comprising good thermalisolating characteristics such that convection between the environmentand the storage volume of the housing sections is reduced and minimised.The system may comprise a centre axis and a symmetry axis, respectively,wherein along the centre axis, the first housing section, the secondhousing section and the plurality of further housing sections may bearranged one after another.

The first housing section has a first storage volume and the secondhousing section has a second storage volume. The first housing sectionand the second housing section are arranged with respect to each othersuch that the first housing section and the second housing section formtogether an overall common storage volume build of the first storagevolume and the second storage volume.

The first opening may be formed at a wall section of the first housingsection. The first opening is adapted for connecting the first storagevolume to the environment. In other words, a working fluid may flow fromthe environment through the first opening into the first storage volume.However, in another operating mode of the system for storing heat,working fluid may flow from the first storage volume through the firstopening into the environment.

Accordingly, the second opening may be formed at a wall section of thesecond housing section. The second opening is adapted for connecting thesecond storage volume to the environment. In other words, a workingfluid may flow from the environment through the second opening into thesecond storage volume. However, in another operating mode of the systemfor storing heat, working fluid may flow from the second storage volumethrough the second opening into the environment.

The first type of first heat storing elements is arranged within thefirst housing section and the second type of second heat storing elementis arranged within the second housing section. The first heat storingelements and the second heat storing elements may be bulk material madeof stones, bricks, lava stone or ceramic parts. The respective heatstoring elements are made of a material with a high heat capacity.

The size of the heat storing elements may be defined by the diameter orthe element volume. Furthermore, the size of the heat storing elementmay be defined by the so called Sauter mean diameter which describes anaverage of the element size of the elements of the (first or second)type. The Sauter mean diameter is defined as the diameter of a spherethat has the same volume/surface area ratio as a heat storing element ofinterest. Because the size for all heat storing elements of one type mayslightly vary, a type may be defined such that at least 80% or more ofheat storing elements of the type have a predetermined desired size.

The above described system for storing heat may be applied in powerplants, in particular in solar power plants, where heat is producedwithout needing a high electric power generation. In this case, heatstoring systems are applied in order to store heat (thermal energy) tilla higher electric power generation is required. Accordingly, theenvironment surrounds the respective openings and the housing sections,respectively. The openings may be coupled to a power plant system whichmay be a heat generating system or heat converting machine or system,respectively.

An operating cycle of an above described storage system is subdividedinto three operating modes:-A charging phase of the storage system—Aworking fluid (such as steam, liquid or gaseous medium) that isgenerated by a heat source, such as a power plant, has a highertemperature than an initial temperature of the heat storing elements.The working fluid is blown into the storage through the respectiveopenings;-A quasi steady-state phase of storage system—A closed systemis present, where no heat exchange with external heat sources exists.The heat storing elements cool down through the natural convection andsystem heat losses; and-A discharging phase—The working fluid isgenerated by a source and has lower temperature then temperature of theheat storing elements. The working fluid is blown into the storagesystem through the respective openings, e.g. from the opposite side incomparison to the charging phase.

The heat storing elements are to be heated (charging process) or cooled(discharging process) up/down to a certain required temperature usingthe working fluid (gas/liquid/mixture). Thus, the storage system workingfluid used for charging and discharging processes is guided through thefirst or second (inlet) openings or respective first or second (outlet)openings of the storage system and is heated up or cooled down by theheat storing elements to the required thermal conditions.

Efficiency of the thermal storage strongly depends on the processdynamics during the charging/discharging phases, special thermo-physicaland geometrical storage and material properties of the heat storingelements, in particular the permeability and thermodynamic boundaryconditions of the heat storing elements.

This means that for the efficiency and the lifespan of the heat storageit is beneficial that the storage heat elements heat up or cooled onconstantly along the whole cross section of the common storage volume.In order to achieve this goal, a mass flow of the working fluid isdivided into a first mass flow which is guidable through the firstopening and into a second mass flow which is guidable through a secondmass. However, if the second housing section is arranged for exampleparticularly above the first housing section, the working fluid movesadditionally vertically inside the common storage volume, e.g. from thefirst storage volume to the second storage volume.

Hence, in order to provide a homogeneous warming or cooling of the firstand second heat storing elements, the first mass flow and the secondmass flow has to be regulated in order to balance and compensate themovement of the thermal energy from the first storage volume to thesecond storage volume.

Hence, by the approach of the present invention, the amount of the firstmass flow of the working fluid and the second mass flow of the workingfluid through the respective openings is controlled by amending the sizeof the respective heat storing elements. For example, in a commonhousing, at least two different types of first and second heat storingelements are arranged at a respective first and second location. Thefirst and second locations may be arranged within a housing section orthe first location may be located in the first housing section and thesecond location may be arranged in the second housing section.Specifically, a first type of first heat storing elements comprises alarger or smaller sized heat storing elements than a second type ofsecond heat storing elements. For example, a flow rate of working fluidthrough larger sized first heat storing elements (with respect tosmaller sized second heat storing elements) may be higher than a flowrate of the working fluid through smaller sized heat storing elements(with respect to larger sized first heat storing elements), and viceversa.

Hence, by predefining the size of the respective heat storing elementsin the first storage volume and the second storage volume, the mass flowof the respective working fluid flowing through the respective first orsecond storage volume and hence the thermal exchange between the firstand second storage volume can be controlled in particular in a passiveway. This means that no active controlling mechanism comprising forexample movable parts is necessary.

According to a further exemplary embodiment, the first housing sectioncomprises a first centre section and a first opening section whichcomprises the first opening, wherein the first type of first heatstoring elements is arranged within the first opening section. Hence,because the first type of first heat storing elements is arranged withinthe opening section, the inlet mass flow or the outlet mass flow of theworking fluid is controllable by the desired and predefined size of thefirst heat storing elements of the first type.

According to a further exemplary embodiment, the first opening sectioncomprises a tapering profile in such a manner that a flow diameter atthe first opening is smaller than at a transition section between thefirst opening section and the first centre section for forming adiffusor section or a nozzle section, respectively. Hence, depending onthe flow direction of the working fluid, the opening section functionsas a diffusor or a nozzle in order to provide a desired flowcharacteristics of the working fluid inside or out of the storagevolume.

Diffusors at the first opening section (charging/fluid inlet side) andconfusors/nozzles (discharging/fluid outlet side) at the further firstopening section of the first housing section improve the working fluiddistribution in the storage medium. Furthermore, first and further firstopening section (i.e. diffusors and confusors) filled with therespective heat storing elements are improving inlet/outlet working flowdistribution of the working fluid.

According to a further exemplary embodiment, the first type of firstheat storing elements is additionally arranged within the first centresection.

According to a further exemplary embodiment, the system furthercomprises a further first type of further first heat storing elements,wherein the further first heat storing elements differ in size and/ormaterial from the first heat storing elements. The further first type offirst heat storing elements is additionally arranged within the firstcentre section. Hence, by the above described embodiment, the first typeof first heat storing elements may be only arranged within the openingsection, whereas in the first centre section, a further type of heatstoring elements, comprising for example a different material and/or adifferent size with respect to the heat storing elements of the firsttype may be arranged.

According to a further exemplary embodiment, the first housing furthercomprises a further first opening for connecting the first storagevolume to the environment, wherein the further first opening is spacedapart from the first opening. Specifically, the further first openingmay be arranged at an opposed side wall section of the first housingsection in comparison to a wall section where the above described firstopening is arranged. More specifically, the first opening may functionas an inlet opening for injecting the working fluid into the firststorage volume and the further first opening may function as an outletopening for exhausting the working fluid out of the first storagevolume.

According to a further exemplary embodiment, the first housing sectioncomprises a further first opening section which comprises the furtherfirst opening. The first type of the first heat storing elements isarranged within the further first opening section. In other words, inthe respective first opening section and in the further first openingsection, the first type of heat storing elements is arranged, wherein inthe centre section of the first housing section, a heat storing elementscomprising a different material and/or size with respect to the heatstoring elements of the first type. However, also a heat storing elementthat differs to the first type and to the further fist type of heatelements is arrangeable within the further first opening section.

According to a further exemplary embodiment, the second housing sectioncomprises a second centre section and a second opening section whichcomprises the second opening. The second type of second heat storingelements is arranged within the second opening section. Hence, becausethe second type of second heat storing elements is arranged within thesecond opening section, the inlet mass flow or the outlet mass flow ofthe working fluid is controllable by the desired and predefined size ofthe second heat storing elements of the second type.

According to a further exemplary embodiment, the second opening sectioncomprises a tapering profile in such a manner that a flow diameter atthe second opening is smaller than at a transition section between thesecond opening section and the second centre section for forming adiffusor section or a nozzle section, respectively.

According to a further exemplary embodiment, the second type of secondheat storing elements is additionally arranged within the second centresection.

According to a further exemplary embodiment, the system comprises afurther second type of further second heat storing elements, wherein thefurther second heat storing elements differ in size and/or material fromthe second heat storing elements. The further second type of second heatstoring elements is additionally arranged within the second centresection. The further second type of second heat storing elements may beadditionally arranged within the second centre section. Hence, by theabove described embodiment, the second type of second heat storingelements may be only arranged within the second opening section, whereasin the second centre section, a further type of heat storing elements,comprising for example a different material and/or a different size withrespect to the heat storing elements of the second type may be arranged.

For example, the further heat storing elements within the first centresection and the second centre section may be made of a similar or samematerial and/or size, for example.

According to a further exemplary embodiment, wherein the second housingsection further comprises a further second opening for connecting thesecond storage volume to the environment, wherein the further secondopening is spaced apart from the second opening. In other words, in therespective second opening section and in the further second openingsection, the second type of heat storing elements is arranged, whereinin the second centre section of the second housing section, heat storingelements comprising a different material and/or size with respect to theheat storing elements of the second type.

By the present invention, first and second openings of the respectivefirst and second housing sections are filled with storage material ofdifferent diameters (i.e. with heat storing elements (particles) ofdifferent size) which leads to different permeability for the workingfluid. Specifically, the material diameters of the heat storing elementsvary over the (vertical) height of the storage. This provides acombination between the partially filled volume of the diffusor(s) andconfusor(s)/nozzle(s) with the storage material and multiple inflowsegmentation over the cross section of the first and second housingsections, taking into account the dynamics of the temperature fieldchanges during charging and discharging phases. The varying heat storingelement (grain) size provides a passive way to regulate the mass flowthrough the different segments of the manifold and hence differentlayers and sections of the storage system.

By the present invention a housing of a storage system comprises along avertical direction a first, a second or a plurality of housing sections.The respective opening sections of the housing sections are partiallyfilled with storage material (heat storing elements). Along the verticaldirection, for every higher-level of opening section (diffusor(s) andnozzle(s)/confusor(s)) and/or for housing sections, respectively, adifferent size of heat storing elements/grains than the (vertically)lower-level diffusor(s) and confusor(s) are used. The element size ofthe used material is dependent on physical and geometrical boundaryconditions of the designed storage system.

On one hand, partially filled diffusor(s) and confusor(s) are improvingthe incoming flow distribution across the storage cross section, on theother hand, usage of different material grain/element sizes, i.e.varying of heat storing permeability in upper and lower-levels of thehousing sections leads to straightening of isotherms along a verticaldirection at a right angle to the horizontal flow of the working fluidin the storage medium during charging and discharging phases of thestorage system. Thus, the straightening of the isotherms improves theefficiency of the designed thermal storage system. The two keyadvantages of the above described invention are the passive nature ofcontrolling the heat distribution during charging and discharging phase(hence no moving or regulated parts) as well as the usage of the storagematerial itself (hence no additional components like manufacturedperforated sheets).

It has to be noted that embodiments of the invention have been describedwith reference to different subject matters. In particular, someembodiments have been described with reference to method type claimswhereas other embodiments have been described with reference toapparatus type claims. However, a person skilled in the art will gatherfrom the above and the following description that, unless othernotified, in addition to any combination of features belonging to onetype of subject matter also any combination between features relating todifferent subject matters, in particular between features of the methodtype claims and features of the apparatus type claims is considered asto be disclosed with this document.

BRIEF DESCRIPTION OF THE DRAWINGS

The aspects defined above and further aspects of the present inventionare apparent from the examples of embodiment to be described hereinafterand are explained with reference to the examples of embodiment. Theinvention will be described in more detail hereinafter with reference toexamples of embodiment but to which the invention is not limited.

FIG. 1 shows a schematical view of a storage system according to anexemplary embodiment of the present invention, wherein the storagesystem is shown in the charging phase; and

FIG. 2 shows a schematical view of a storage system according to anexemplary embodiment of the present invention, wherein the storagesystem is shown in the discharging phase.

DETAILED DESCRIPTION

The illustration in the drawings is in schematic form. It is noted thatin different figures, similar or identical elements are provided withthe same reference signs.

FIG. 1 and FIG. 2 show a schematic view of a storage system 100according to an exemplary embodiment of the present invention. Thesystem 100 comprises a first housing section 110 with a first storagevolume V1, wherein the first housing comprises a first opening 113 forconnecting the first storage volume V1 to an environment. A first type111 of first heat storing elements is arranged within the first housingsection 110. The system further comprises a second housing section 120with a second storage volume V2, wherein the second housing section 120comprises a second opening 123 for connecting the second storage volumeV2 to the environment, wherein the first storage volume V1 and thesecond storage volume V2 are interconnected for forming a common storagevolume. A second type 121 of second heat storing elements is arrangedwithin the second housing section 120. The first heat storing elementsof the first type 111 comprise a respective first element size, whereinthe second storing elements of the second type 121 comprise a respectivesecond element size. The first element size differs from the secondelement size.

The system 100 comprises the housing which is formed of the firsthousing section 110, the second housing section 120 and the thirdhousing section 130. The first housing section 110, the second housingsection 120 and the third housing section 130 may comprise arectangular, elliptical cross section. The housing sections 110, 120,130 may be made of metal material, for example. The housing sections110, 120, 130 are surrounded by an environment, wherein the housingsections 110, 120, 130 comprise material and/or an additional insulationlayer comprising good thermal isolating characteristics such thatconvection between the environment and the storage volume of the housingsections 110, 120, 130 is reduced and minimized. The system 100 maycomprise a center axis 103 and a symmetry axis, respectively, whereinalong the center axis 103, the first housing section 110, the secondhousing section 120, the third housing sections 130 and/or a pluralityof further third housing sections 130 may be arranged one after another.The system 100 is formed such that the center axis 103 has at least onecomponent which is parallel to the vertical direction.

The first housing section 110 has a first storage volume V1, the secondhousing section 120 has a second storage volume V2 and the third housingsection 120 has a third storage volume V3. The first housing section110, the second housing section 120 and the third housing section 130are arranged with respect to each other such that the first housingsection 110, the second housing section 120 and the third housingsection 130 form together an overall common storage volume build of thefirst storage volume V1, the second storage volume V2 and the thirdstorage volume V3.

The first opening 113 is formed at a wall section of the first housingsection 110. The first opening 113 is adapted for connecting the firststorage volume V1 to the environment. The second opening 123 is formedat a wall section of the second housing section 120. Accordingly, thesecond opening 123 is adapted for connecting the second storage volumeV2 to the environment. The third opening 133 is formed at a wall sectionof the third housing section 130. The third opening 133 is adapted forconnecting the third storage volume V3 to the environment.

Hence, a total inlet flow 101 is separated in a first inlet flow 118which is guided through the first opening 113 into the first storagevolume V1, in a second inlet flow 128 which is guided through the secondopening 123 into the second storage volume V2, and in a third inlet flow138 which is guided through the third opening 133 into the third storagevolume V3.

The first type 111 of first heat storing elements is arranged within thefirst housing section 110, the second type 121 of second heat storingelements is arranged within the second housing section 120 and the thirdtype 131 of third heat storing elements 130 is arranged within the thirdhousing section 130.

As shown in FIG. 1 and FIG. 2, the respective housing sections 110, 120,130 comprise respective center sections 114, 124, 134. Furthermore, therespective housing sections 110, 120, 130 comprise respective openingsections 112, 122, 132 which comprise the respective openings 113, 123,133.

In the exemplary embodiment shown in FIG. 1 and FIG. 2, the first type111 of first heat storing elements is arranged within the first openingsection 112, the second type 121 of second heat storing elements isarranged within the second opening section 122 and the third type 131 ofthird heat storing elements is arranged within the third opening section132.

Hence, because e.g. the first type 111 of first heat storing elements isarranged within the first opening section 112, the first inlet mass flow118 (or the first outlet mass flow respectively 119) of the workingfluid is controllable by the desired and predefined size of the firstheat storing elements of the first type 111.

Furthermore, the second type 121 of second heat storing elements isarranged within the second opening section 122, the second inlet massflow 128 (or the second outlet mass flow respectively 129) of theworking fluid is controllable by the desired and predefined size of thesecond heat storing elements of the second type 121. Accordingly, thethird type 131 of third heat storing elements is arranged within thethird opening section 132, the third inlet mass flow 138 (or the thirdoutlet mass flow respectively 139) of the working fluid is controllableby the desired and predefined size of the third heat storing elements ofthe third type 131.

The respective opening sections 112, 122, 132 comprise e.g. a taperingprofile in such a manner that a flow diameter at the respective opening113, 123, 133 can be smaller than at a transition section between therespective opening sections 112, 122, 132 and the respective centersections 114, 124, 134 for forming a diffusor section or a nozzlesection, respectively. Hence, depending on the flow direction of theworking fluid, the respective opening sections 112, 122, 132 function asa diffusor or a nozzle in order to provide a desired flow characteristicof the working fluid inside or out of the respective storage volume V1,V2, V3.

Furthermore, as shown in FIG. 1 and FIG. 2, the first housing 110further comprises a further first opening 116 for connecting the firststorage volume V1 to the environment, wherein the further first opening116 is spaced apart from the first opening 113. Specifically, thefurther first opening 116 may be arranged at an opposed side wallsection of the first housing section 110 in comparison to a wall sectionwhere the first opening 113 is arranged. More specifically, in acharging phase as shown in FIG. 1, the first opening 113 may function asan inlet opening for injecting the first inlet flow 118 of the workingfluid into the first storage volume V1 and the further first opening 116may function as an outlet opening for exhausting the working fluid in afirst outlet flow 119 out of the first storage volume V1.

Accordingly, the second housing 120 and the third housing section 130further comprise respective further openings 126, 136 for connecting therespective storage volumes V2, V3 to the environment, wherein thefurther openings 126, 136 are spaced apart from the respective openings123, 133. Specifically, the further openings 126, 136 may be arranged atan opposed side wall section of the respective housing sections 120, 130in comparison to a wall section where the respective opening 123, 133are arranged. More specifically, in a charging phase as shown in FIG. 1,the openings 123, 133 may function as respective inlet opening forinjecting the respective inlet flows 128, 138 of the working fluid intothe respective storage volumes V2, V3 and the further openings 126, 136may function as respective outlet opening for exhausting the workingfluid in respective outlet flows 129, 139 out of the respective storagevolumes V2, V3.

As shown in FIG. 1 and FIG. 2, the respective housing sections 110, 120,130 comprise respective further opening sections 117, 127, 137 whichcomprise the respective further openings 116, 126, 136. The respectivetypes 111, 121, 131 of the respective heat storing elements orrespective further types 115, 125, 135 of further heat storing elementsare arranged within the respective further opening sections 117, 127,137. In other words, in the respective opening sections 112, 122, 132and in the respective opening sections 117, 127, 137, the respectivetypes 111, 121, 131 of heat storing elements are arranged, wherein inthe respective center sections 114, 124, 134 of the respective housingsection 110, 120, 130, a further type of heat storing elementscomprising a different material and/or size with respect to the heatstoring elements of the respective type is arranged.

In exemplary embodiments of the present invention, various types of heatstoring elements 111, 121, 131 and respective further types 115, 125,135 of further heat storing elements are arrangeable in the differenthousing sections 110, 120, 130. For example, one of the housing sections110, 120, 130 comprises the further type 115, 125, 135 of further heatstoring elements located within the respective centre sections 114, 124,134, wherein the further heat storing elements of the further type 115,125, 135 comprises a respective further element size. The furtherstoring element size 115, 125, 135 may differ from the first elementsize of the first heat storing elements of the first type 111, 121, 131(which may be located within the respective opening sections 112, 122,131) and/or from an additional further first element size of additionalfurther heat storing elements of an additional further type 111, 121,131 (which may be located within the respective further opening sections117, 127, 137).

Accordingly, the first housing section 110 may comprise three differentfirst types 111, 115 of heat storing elements in the first openingsection 112, the first centre section 114 and the further first openingsection 117.

The second housing section 120 may comprises also three different secondtypes 121, 125 of heat storing elements in the second opening section122, the second centre section 124 and the further second openingsection 127. The three different second types 121, 125 of heat storingelements may have the same size with respect to the three differentfirst types 111, 115. Hence, first and second types 111,115, 121, 125with three different sizes are used in the first and second housingsections 110, 120.

Alternatively, the three different second types 121, 125 of heat storingelements may have a different size with respect to the three differentfirst types 111, 115. Hence, first and second types 111,115, 121, 1125with six different sizes are used in the first and second housingsections 110, 120.

Accordingly, the third housing section 130 or additional further hosingsections may be filled with respective types of heat storing elements inthe same manner as described above for the first housing section 110 andthe second housing section 120.

An operating cycle of the storage system 100 is subdivided into threeoperating modes. In a charging phase of the storage system 100 as shownin FIG. 1, respective inlet flows 118, 128, 138 of a hot working fluid(such as steam, liquid or gaseous medium) that is generated by a heatsource, such as a power plant, has a higher temperature than an initialtemperature of the heat storing elements. The working fluid is blowninto the storage through the respective openings 113, 123, 133. Afterheating the heat storing elements, the working fluid is blown out of thestorage system 100 through the respective further openings 116, 126,136.

The mass flows of the working fluid through the respective storagevolumes V1, V2, V3 are controlled by the sizes of the respective heatstoring elements of the respective types 111, 121, 131 and further types115, 125, 135 of the heat storing elements.

In a quasi-steady-state phase of storage system 100, a closed system ispresent, where no heat exchange with external heat sources exists and nomass flow through the storage system 100 exists. The heat storingelements cool down through the natural convection and system heatlosses.

In a discharging phase of the storage system 100 as shown in FIG. 2, theworking fluid flowing e.g. in opposite direction with respect to thecharging phase, flows in respective inlet flows 118, 128, 138 into thestorage through the respective further openings 116, 126, 136. Theworking fluid has lower temperature than the temperature of the heatstoring elements. After heating the working fluid and after cooling downthe heat storing elements, respectively, the working fluid is blown outof the storage system 100 through the respective openings 113, 123, 133.

The respective mass flow of the working fluid through the respectiveopenings 113, 123, 133 and further openings 116, 126, 136 is controlledby amending the size of the respective heat storing elements of therespective types 111, 121, 131 and further types 115, 125, 135. Hence,by predefining the size of the respective heat storing elements inrespective storage volumes V1, V2, V3 and/or the respective openingsections 112, 122, 132 and further opening sections 117, 127, 137, themass flow of the respective working fluid flowing through the respectivestorage volumes V1, V2, V3 and hence the thermal exchange between therespective storage volumes V1, V2, V3 can be controlled in particular ina passive way.

It should be noted that the term “comprising” does not exclude otherelements or steps and “a” or “an” does not exclude a plurality. Alsoelements described in association with different embodiments may becombined. It should also be noted that reference signs in the claimsshould not be construed as limiting the scope of the claims.

1. A system for storing heat, the system comprising a housing with astorage volume, wherein the housing comprises an opening for connectingthe storage volume to an environment, a first type of first heat storingelements which is arranged within a first location within the storagevolume, a second type of second heat storing elements which is arrangedwithin a second location within the storage volume, wherein the firstheat storing elements of the first type comprise a respective firstelement size, wherein the second storing elements of the second typecomprise a respective second element size, and wherein the first storingelement size differs from the second element size.
 2. The systemaccording to claim 1, wherein the housing section further comprises afurther opening for connecting the storage volume to the environment,wherein the further opening is spaced apart from the opening, whereinthe first type of first heat storing elements is arranged within theopening forming the first location, and wherein the second type ofsecond heat storing elements is arranged within the further openingforming the second location.
 3. The system according to claim 2, whereinthe housing section further comprises a center section arranged betweenthe opening and the further opening, wherein an additional further typeof additional further heat storing elements is located within the centresection, wherein the additional further heat storing elements of theadditional further type comprise a respective additional further elementsize, and wherein the additional further storing element size differsfrom the first element size and/or the second element size.
 4. Thesystem according to claim 1, wherein the housing comprises a firsthousing section and the storage volume comprises a first storage volume,wherein the first housing section comprises a first opening forming theopening for connecting the first storage volume to the environment,wherein the first type of first heat storing elements is arranged withinthe first housing section forming the first location, wherein thehousing further comprises a second housing section and the storagevolume comprises a second storage volume, wherein the second housingsection comprises a second opening for connecting the second storagevolume to the environment, wherein the first storage volume and thesecond storage volume are interconnected for forming a common storagevolume, and wherein the second type of second heat storing elements isarranged within the second housing section forming the second location.5. The system according to claim 4, wherein the first housing sectioncomprises a first centre section and a first opening section whichcomprises the first opening, wherein the first type of first heatstoring elements is arranged within the first opening section.
 6. Thesystem according to claim 5, wherein the first opening section comprisesa tapering profile in such a manner that a flow diameter at the firstopening is smaller than at a transition section between the firstopening section and the first centre section for forming a diffusorsection or a nozzle section, respectively.
 7. The system according toclaim 5, wherein the first type of first heat storing elements isadditionally arranged within the first centre section.
 8. The systemaccording to claim 5, further comprising a further first type of furtherfirst heat storing elements, wherein the further first heat storingelements differ in size and/or material from the first heat storingelements, and wherein the further first type of first heat storingelements is additionally arranged within the first centre section. 9.The system according to claim 1, wherein the first housing sectionfurther comprises a further first opening for connecting the firststorage volume to the environment, wherein the further first opening isspaced apart from the first opening.
 10. The system according to claim1, wherein the first housing section comprises a further first openingsection which comprises the further first opening, and wherein the firsttype of the first heat storing elements is arranged within the furtherfirst opening section.
 11. The system according to one of the claims 1to claim 1, wherein the second housing section comprises a second centersection and a second opening section which comprises the second opening,and wherein the second type of second heat storing elements is arrangedwithin the second opening section.
 12. The system according to claim 11,wherein the second opening section comprises a tapering profile in sucha manner that a flow diameter at the second opening is smaller than at atransition section between the second opening section and the secondcentre section for forming a diffusor section or a nozzle section,respectively.
 13. The system according to claim 11, wherein the secondtype of second heat storing elements is additionally arranged within thesecond centre section.
 14. The system according to claim 11, furthercomprising a further second type of further second heat storingelements, wherein the further second heat storing elements differ insize and/or material from the second heat storing elements, and whereinthe further second type of second heat storing elements is additionallyarranged within the second centre section.
 15. The system according toclaim 1, wherein the second housing section further comprises a furthersecond opening for connecting the second storage volume to theenvironment, wherein the further second opening is spaced apart from thesecond opening, and wherein a further second type of further second heatstoring elements is arranged within the further second opening.
 16. Amethod for manufacturing a system for storing heat, the methodcomprising providing a first housing section with a first storagevolume, wherein the first housing comprises a first opening forconnecting the first storage volume to an environment, providing a firsttype of first heat storing elements, arranging the first type of firstheat storing elements within the first housing section, providing asecond housing section with a second storage volume, wherein the secondhousing section comprises a second opening for connecting the secondstorage volume to the environment, wherein the first storage volume andthe second storage volume are interconnected for forming a commonstorage volume, providing a second type of second heat storing elements,and arranging the second type of second heat storing elements within thesecond housing section, wherein the first heat storing elements of thefirst type comprise a respective first heat storing element size,wherein the second heat storing elements of the second type comprise arespective second element size, and wherein the first element sizediffers from the second element size.